Minmax-optimal list searching with $O(\log_2\log_2 n)$ average cost

Abstract: We find a searching method on ordered lists that surprisingly outperforms binary searching with respect to average query complexity while retaining minmax optimality. The method is shown to require $O(\log_2\log_2 n)$ queries on average while never exceeding $\lceil \log_2 n \rceil$ queries in the worst case, i.e. the minmax bound of binary searching. Our average results assume a uniform distribution hypothesis similar to those of prevous authors under which the expected query complexity of interpolation search of $O(\log_2\log_2 n)$ is known to be optimal. Hence our method turns out to be optimal with respect to both minmax and average performance. We further provide robustness guarantees and perform several numerical experiments with both artificial and real data. Our results suggest that time savings range roughly from a constant factor of 10\% to 50\% to a logarithmic factor spanning orders of magnitude when different metrics are considered.